Note: Descriptions are shown in the official language in which they were submitted.
~ n ~ 37 6 3
CTMP-Process
The present invention relates to an absorbent chemi-
thermomechanical pulp and to a method of manufacturing
the same.
Hitherto, it has only been possible to apply the pro-
cess of defibering chips with a low energy input subse-
quent to preheating the chips under high pressure and
high temperature (150-170~C), the so-called Asplund
process, within the board manufacturing industry, since
the pulp resulting from this process is dark in colour
and cannot be bleached at reasonable chemical consump-
tions. Furthermore, the fibres become coated with a
lignin skin and are therefore stiff and rigid, which
results in poorer strength and absorption properties.
Consequently, it has only been possible to produce
chemithermomechanical pulp (CTMP) of high brightness
and good absorbency by preheating and refining at a
temperature of at most 140~C. High brightness is espe-
cially important when producing tissue pulp.
DE-A-27 14 730 describes a process for producing a
chemically modified thermomechanical pulp where the
wood material is preheated at a temperature of 135-
200~C during 1-30 minutes. The time used according to
the examples is of the order of 10 minutes; To obtain
the desired flexibility an energy input of twice the
normal is required.
The object of the present invention is to provide a
chemithermomechanical pulp which exhibits a low resin
content, an extremely high long-fibre content, an
extremely low short-fibre content, and an extremely low
shive content. Such pulps are particularly suited for
the manufacture of fluff and tissue. The extremely low
~ t ~
2 t~ 7 3 7 6 3
1 a
shives content is of special importance when producing
tissue pulp. The extremely high long-fiber content with
the corresponding high freenes is of special importance
when producing fluff pulp.
A further object of the invention is to provide a novel
method for the manufacture of absorbent chemithermo-
E s~EEr
~ ~ ~ 3 7 6 3
mechanlcal pulps at low energy lnputs.
The lnvention thus relates to an absorbent cheml-
thermomechanlcal pulp produced from llgnocelluloslc materlal
at a wood yleld above 88%, a resln content beneath 0.15%,
calculated on the amount of resln whlch can be extracted ln
dlchloromethane, a hlgh long-flbre content, a low short-flbre
content and a low shlve content, the pulp belng characterlzed
ln that when fractlonatlng the pulp accordlng to Bauer McNett,
the long-flbre content ls above 70%, preferably above 75%,
especlally 78%, of flbres retalned on a wlre gauge of slze 28
mesh and the short-flbre content ls beneath 10%, preferably
beneath 8%, especlally 6% of flbres whlch pass through a wlre
gauze of slze 200 mesh accordlng to Bauer McNett; and ln that
the shlve content ls lower than 3%, preferably lower than 2%,
measured accordlng to Sommervllle.
The pulp should have such brlghtness that lt can be
bleached at a reasonable consumptlon of bleachlng chemlcals to
a brlghtness of at least 65% IS0, preferably 70%.
Alternatlvely the pulp may have been bleached to such
brlghtness.
Thls pulp ls partlcularly well sulted for the
manufacture of fluff and tlssue.
When the pulp ls a fluff pulp lt ls preferably
reflned to a freeness of 740 ml at the lowest, especlally 750
ml at the lowest and sultably 760 ml CSF at the lowest. Such
a pulp does not need to be bleached and may have a brlghtness
of at least 45% IS0.
When the pulp ls a tlssue pulp lt has sultably a
20615-956
B
WO9l/12~7 PCT/SE91/00091
- 2073763~'
brightness of at least 65 % ISO, preferably above 70 %.
The tissue pulp does not need to have as high a freenes
as the fluff pulp. Suitably it is refined to a freenes
of 650 ml CSF at the lowest.
The problem with manufacturing pulp suitable for fluff
and tissue by means of a chemithermomechanical method
lies in the desired combination of high freeness, high
long-fibre content, low shive content and high bright-
ness. An increase in temperature when preheating will
favour the reduction in shive content but, at the same
time, impair brightness.
It has now surprisingly been found that a chemither-
momechanical pulp having the desired properties can be
produced by
a) impregnating the chips with sodium sulphite, sodium
dithionate, alkaline peroxide or the like, with an
addition of a complex builder;
b) preheating the chips;
c) defibering the chips to pulp in a refiner at sub-
stantially the same pressure and temperature as those
employed in the preheating process; and
d) washing and dewatering the pulp to, e.g., a con-
sistency of 25-50%,
wherein, in accordance with the invention, impregnation
and preheating of the chips is effected in one and the
same vessel over a combined treatment time of at most 2
minutes, particularly at most 1 minute, preferably at
most 0.5 minute; and
a) using a warm impregnating liquid having a tempera-
ture of at least 100~C, suitably at least 130 C and
preferably having essentially the same temperature as
that of the preheating process;
WO91/12367 PCT/SE91/00091
.
2073763 ~
b) preheating the chips at a temperature of 150-175 C,
preferably 160-170 C; and
c) carrying out the defibering process with an energy
input which is at most half of the energy input re-
quired for defibering to the same shive content in a
similar refiner when preheating and defibering are
performed at 135 C.
The complex builder used in the impregnating process
may, for instance, be DTPA, which contributes to an
improvement in pulp brightness.
The pulp may e) be refined to a brightness above 65 %
ISO, preferably above 70 %. To accomplish this at a
reasonable consumption of bleaching chemicals the
brightness after refining has to be at least 45 % IS0,
preferably at least 50 %. Such bleaching should prefer-
ably be performed when the pulp is a tissue pulp.
In order to obtain a pulp of sufficient brightness, it
is essential that preheating at the aforesaid high
temperature is not permitted to proceed over a period
of time of as long a duration as the standard prehea-
ting time of about 3 minutes used when producing chemi-
mechanical pulp of CTMP type. In order to enable the
preheating time to be lowered to at most 2 minutes,
preferably at most l minute, it is necessary to use an
impregnating solution which is heated to a temperature
of at least 100~C, particularly at least 130 C and
preferably substantially to the same temperature as
that used in the preheater. Furthermore, no impregna-
ting liquid shall be removed between the impregnating
and preheating steps. Consequently, impregnation is
effected in the same vessel as that in which the chips
are preheated, and at the same pressure and suitably at
WO91/12367 PCT/SE91/00091
20.7..3763
,
the same temperature or only a slightly lower tempera-
ture. The brightness of the pulp is sustained because
of the very short stay time at the high temperature, so
that an excessively large quantity of bleaching chemi-
cals, ssuch as peroxide, will not be required in the
following bleaching step. Furthermore, the wood yield
obtained in this way is almost equal to the wood yield
obtained when preheating the chips conventionally at
130-140 C. In addition, when refining to a freeness
slightly above 750 ml CSF, the energy input required
for the defibering process is reduced from about 600
kWh/tonne at 130 C to less than 300 kWh/tonne at 170 C.
These values have been obtained in a pilot plant.
Commersial values may differ from those obtained at
pilot level. The relative differences between the
levels for shives content, brightness and energy input
obtained in the pilot plant at conventional temperature
and at the temperature according to the invention,
respectively, should, however, remain in a commercial
plant.
The inventive method suitably includes the conventional
steaming, impregnating, preheating, defibering, wash-
ing, screening, washing, possibly bleaching, washing
and drying stages. Whereas a conventional impregnating
process is carried out with cold liquid in a vessel
other than the preheating process, which is carried out
over a period of about 3 minutes and at a temperature
of about 130 C, and in which process impregnating
liquid is removed between the impregnating stage and
the preheating stage, the impregnating and preheating
processes of the inventive method are combined in one
and the same vessel and are carried out at the same
pressure and substantially the same temperature 100-
175 C, 150-175 C respectively, over a combined time
WO91/12367 PCT/SE91/00091
'20i3763
period of at most 2 minutes, suitably at most 1 minute
and preferably at most 0.5 minute.
Because preheating is effected at high temperature, the
refining process requires less energy. A low energy
input will normally result in high freeness and high
shive content. A surprising characteristic of the
present invention is that at low energy inputs, success
is achieved in combining high freeness with low shive
content. Low energ~ input would otherwise result in a
high shive conten~
When applyin~.the inventive method in tests on a
laboratory seal, a freeness of above 780 ml CSF was
achieved with an acceptable shive content. In some
instances, a freeness of above 800 ml was achieved.
This can be compared with a freeness of about 650-750
ml CSF in the normal production of CTMP-fluff.
The pulp is washed subsequent to the refining process,
suitably under pressure and at high temperature, pre-
ferably while excluding air from the system and in
immediate connection with the refining stage. The pulp
is dewatered to a consistency of e.g. 25-50%. Possible
bleaching is then carried out with peroxide or other
bleaching chemical. If desired, the pulp can again be
washed, after the bleaching process.
When producing fluff, defibering is carried out to a
freeness of 740 ml at the lowest, suitably of 750 at
the lowest, preferably of 780 ml CSF at the lowest.
When producing tissue pulp the refining may be carried
out to a freenes of 650 ml CSF at the lowest.
WO91/12367 PCT/SE91/00091
2073763
When applying the inventive method, it is possible to
produce pulp with a wood yield above 88~, preferably
above 90~, a resin content of less than 0.15%,
calculated on the amount of resin that can be extracted
in dichloromethane, and a brightness above 65% IS0
after bleaching.
The invention will now be described in more detail with
reference to the following exemplifying embodiments
thereof and with reference to the accompanying draw-
ings, in which
Figure 1 illustrates schematically a test plant used in
the exemplifying embodiments;
Figure 2 is a diagram showing shive content against
energy input at defibering;
Figure 3 is a diagram showing energy at defibering
against preheating temperature;
Figure 4 is a diagram showing long-fibre content
against energy input at defibering;
Figure 5 is a diagram showing short-fibre content
against energy input at defibering;
Figure 6 is a diagram showing network strength against
energy input;
Figure 7 is a diagram showing peroxide consumption
against original brightness after defibering;
Figure 8 is a diagram showing brightness after defiber-
ing against peroxide consumption; and
Figure 9 is a diagram showing fibre length against
energy input after defibering.
Figure 10 is a diagram showing the brightness obtained
after defibering against preheating temperature; and
Figure 11 is a diagram showing brightness after defi-
bering against preheating temperature.
WO91/12367 PCT/SE91/00091
2073763
In order to study the possiblity of manufacturing fluff
and tissue pulp in a high-temperature variant of a
CTMP-process, there was used a test plant schematically
illustrated in Figure 1. The plant was constructed so
that the pulps could be washed in immediate connection
with refining at high temperature.
The chips are introduced into the preheater 2 with the
aid of the feed screw 1 and are impregnated at the
preheater inlet. The preheated chips are then passed
immediately to the refiner 3, where the chips are
defibered while supplying water. When starting-up the
plant, and when samples-shall be taken immediately
after the refining stage, the resultant pulp is passed
to the cyclone 4 where samples can be taken in the
direction of arrow 5. The connecting line to the cy-
clone 4 is then disconnected and the blower line 6
connected instead, such as to thin the pulp to a con-
sistency of about 3% during transportation to a vessel
7 equipped with a pump which functions as a mixer. The
pulp is then pumped to a level vessel 8 which is con-
nected directly to a screw press 9. The entire system,
from impregnation to dewatering in the screw press, can
be pressurized to 1 MPa.
Spruce sawmill chips were used in the tests. The chips
were screened on two different screens, to remove
excessively coarse chips and sawdust. The screens had a
hole diameter of 35 mm and 8 mm respectively. The chips
were impregnated with 50 kg sodium sulphite and 3 kg
DTPA per tonne of chips in all tests, prior to the
preheating, refining and washing stages.
WO91/12367 PCT/SE91/00091
2073763
Example
Chips were treated in the plant shown in Figure 1 at
different temperatures during the preheating-refining
process. The temperature was allowed to vary between
135 and 170~C. The impregnating liquid was subjected to
a heat exchange and brought to the temperature level of
the preheater. At each temperature level in the re-
finer, the pulp was washed at a temperature of about
10 C beneath the preheated temperature and at a tem-
perature of about 90~C under atmospheric pressure. The
stay time in the preheater was maintained as constant
as possible over a period of about 1 minute.
Subsequent to impregnation with the same chemical input
as that used for remaining pulps, a CTMP-pulp was
produced in an OVP-20 (Open Vertical Preheater) at a
preheating and refining temperature of 135 C, this pulp
being used as a reference pulp.
The results of the tests carried out on the pulps are
shown partly in Figures 2-9 and in the following Table.
These show typical results obtained in this pilot plant
for some of the parameters of interest for the inven-
tion.
The following Table I shows some of the results ob-
tained.
WO 91/12367 PCr/SE91/00091
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WO9l/12367 PCT/SE91/~91
11 2073~763 ' ~:
Tests were also carried out at laboratory level in a 10
litre digester. The chips were steamed at atmospheric
pressure and then impregnated with a weak alkaline sul-
phite solution before the pressurized steam treatment
at high temperature.
500 g of spruce chips with a dry solids content of
48.1% were steamed at a temperature of 100 C over a
period of 2 minutes. The impregnating solution con-
tained 20 g/l sodium sulphite and 3.2 g/l DTPA and had
a temperature of 100~C. The impregnation was carried
out for 1 minute under a nitrogen pressure of 7 bar.
After removal of excess impregnating solution the chips
were heated to their respective heating temperatures as
fast as possible. Condensate was drained while heating.
The time at each temperature was varied. Thereafter the
chips were cooled in cold water. These chips were then
refined and tested for brightness.
The results obtained are shown in the following Table
II and on the Figures 10 and 11.
.
WO91/12367 PCT/SE91/00091
' 2073~763 12
Table II Analysis Data
PreheatingPreheating Sample Brightness
temperature C time, min. K 21/90 % ISO
0 58.3
58.3
~l 58.3
135 ' 2 A2 62.6
A5 58.2
150 1/2 B1 60.7
2 B2 60.0
B5 54.1
160 2 C2 54.5
C5 49.5
170 1/2 D1 54.1
2 D2 51.3
D5 46.6
